Americans have been hearing for years that their kids are lagging behind the rest of the developed world in skills. Now it's the adults' turn for a reality check.

A first-ever international comparison of the labor force in 23 industrialized nations shows that Americans ages 16 to 65 fall below international averages in basic problem-solving, reading and math skills, with gaps between the more- and less-educated in the USA larger than those of many other countries.

The findings, out Tuesday from the U.S. Department of Education, could add new urgency to U.S. schools' efforts to help students compete globally.

The new test was given to about 5,000 Americans between August 2011 and April 2012. The results show that the typical American's literacy score falls below the international average, with adults in 12 countries scoring higher and only five (Poland, Ireland, France, Spain and Italy) scoring lower. In math, 18 countries scored higher, with only two (Italy and Spain) scoring lower. In both cases, several countries' scores were statistically even with the USA.

The oldest Americans in the sample turned in a higher-than-average performance in reading, with 9% of test-takers between 55 and 65 years old scoring at the top proficiency level, compared to just 5% worldwide. In math, however, they were even with the 7% international average.

The problem, the new findings suggest, is with younger U.S. workers, who lag in nearly every category.

The results are "quite distressing," says Harvard University's Paul Peterson, co-author of Endangering Prosperity, a recent book on education and international competitiveness. "Other countries have been catching up for some time," he says. "At one time, we had a really significant lead, but those people are disappearing from the workforce."

"Adults who have trouble reading, doing math, solving problems and using technology will find the doors of the 21st century workforce closed to them," Education Secretary Arne Duncan says. "We need to find ways to challenge and reach more adults to upgrade their skills."

Other findings:

• Average literacy scores ranged from 250 in Italy to 296 in Japan. The U.S. average: 270.

• Average math scores ranged from 246 in Spain to 288 in Japan. The U.S. average: 253.

• Only 9% of U.S. adults performed at the highest proficiency level on math. Just three countries — South Korea, Italy and Spain — had a lower average.

• Overall, about one in eight Americans turned in a top performance in reading — seven countries had a higher percentage of top performers. In math, 15 countries had more top performers.

What brought the U.S. average down was a larger-than-average gap in skills between groups, such as those with or without a college degree, and between workers whose jobs do or don't require advanced math and reading skills.

While those gaps may not show up immediately in productivity totals, Peterson says, in time they'll have an effect. "There's a 20-year delay between the quality of the educational system and its impact," he says. "It's sort of like watching a car crash in slow motion."